[17/N] Desks: Adjust activity opacity calculation

Checks whether the container is an ActivityRecord and applies the
same opacity calculation it used prior to
I166fa4977d6c1e988275d58ea718ce1119ab9b66, otherwise activities will
always be considered transparent.

Flag: com.android.window.flags.enable_multiple_desktops_backend
Bug: 395755508
Test: manual - activities or tasks behind Home/Launcher are
invisible/paused
Test: atest ActivityTaskSuperVisorTests

Change-Id: I7bdf3839003af6bec1d586b17ed82c72dc9c0642